The Bettas often don't like any snails or other fish in "their" territory. That said sometimes they do harass them some then leave them alone, and sorry to say, sometimes they will bite the unsuspecting snail, injuring and/or killing them.
My Betta was flaring at my RH snail and doing what I call stomping, since they dont' have feet (lol) it is like when a school yard bully would stomp out his foot at his intended target threatening the target to menace /goad him.
The Betta jerk towards the target just to the point of biting, up to and including bumping into them. Otherwise I would call the biting an attack and not allow them to be together.
You can use a very large vase, with an air bubbler ideally. Pwc on a probably daily basis.
Or there are needlepoint plastic mesh sheets which run about .79-1.59 and you might be able to make that into a separation screen, the snail doen't need a huge side of the tank. You can sew them together with fishing line, just whip stiich and tie the line every inch or 2, and /or double stitch up and then down.
Make sure that the mesh goes into the substrate enough to cover the crack. Betta are able to shimmy through tiny places, as well make it as high as you can above the tanks waterline, I have seen mine jump right over the divider like a pole vaulter (or shiimy in the sand under the divider) to get to the other fish/thing on the other side.